The present invention relates to method and apparatus for the production of semiconductor ribbon from a melt and more particularly to improved apparatus and method in which ribbon stress and brittleness are avoided by controlling the temperature profile of a moving chill surface.
A reference known to the Applicants and believed to be relevant to the present invention is U.S. Pat. No. 3,605,863 issued to King on Sept. 20, 1971. This patent is hereby incorporated by reference for its general teachings of apparatus and methods for formation of ribbon materials which are generally applicable to formation of semiconductor ribbons.
It is generally recognized that a major portion of the cost of production of solar cells is the cost of the original high-purity silicon and its preparation into wafers of suitable size and shape. A standard process involves the growing of monocrystalline boules using the Czochralski technique and then mechanically sawing the boules into thin slices or wafers. Half of the original silicon material is typically lost in a sawing process. Photovoltaic cells have also been made from polycrystalline silicon after it was poured into essentially brick-shaped molds and then sawed into square slices. While such processing of polycrystalline material is less expensive than growth of single-crystal material, the resulting cells have lower efficiency and essentially half of the material is still lost in the sawing process. Much effort has therefore been made to develop apparatus and techniques for producing wafers or ribbons of silicon directly from the molten state. The above-referenced King patent illustrates one type of apparatus which has resulted from such efforts.
The basic method illustrated by the King patent includes the contacting of a molten body of material with the surface of a moving drum or wheel. The surface of the wheel is cool relative to the molten mass and by cooling a portion thereof pulls or drags off a film or ribbon of the material in a solidified or partially-solidified form. When such a process is used to manufacture ribbons of metallic materials good quality ribbons can be obtained over a fairly wide range of operating conditions. However, the application of the method to production of semiconductor materials has proven difficult for a number of reasons including the fact that semiconductor materials undergo a considerable dimensional change upon solidification. The dimensional change results in considerable internal stress which typically causes waviness in the produced material or can cause it to spontaneously split or even shatter into small pieces. While various annealing techniques have been employed to remove the built-in stress after ribbon formation it would be very desirable to eliminate the stress from the material as produced.